iMonitorSubject is the simplest and most common form of Visual Host.  All visual field instances implement this interface.  It has two methods that will be called depending on how the field is used.

ApplyMonitoredValue is typically called when the field is used as the target of a monitor.  A reference to the Monitor Source component is received in the MonitorSubject input map.

GetValue can be called to obtain the value of the field.  As GetValue applies to any field type, the result map is a variant.

The ValueChanged event that can be signaled to indicate that the value of the variable has changed.  This equates to the Changed event.